1. Field of Invention
The present invention relates to a light guide plate which is supplied with light sideways and deflects the light to output from an emission face, further relating to a surface light source device employing the light guide plate, still further relating to a liquid crystal display employing the surface light source device for back-lighting or front-lighting.
2. Related Art
A surface light source device of a type comprises a light guide plate having an end face, through which light is introduced, and two major faces (i.e. faces larger than end faces) one of which provides an emission face, being employed for various uses such as back-lighting or front-lighting for a liquid crystal display. Basic performance of surface light source devices of such a type greatly depends on light guide plates employed therein.
A basic function of a light guide plate is to change a propagation direction (roughly in parallel with an emission face of the light guide plate) of light introduced into the light guide plate through a side end face so that the light is emitted from the emission face. As known well, a simply transparent light guide plate to which no modification is applied is capable of deflecting light slightly, providing an unsatisfactory brightness. Therefor any means for promoting emission from the emission face is required.
Means for promoting emission from a light guide plate relies upon one of the followings or some of them as combined.
(1) Scattering power within a light guide plate (light scattering guide plate);
(2) Emission face (a major face) provided with light diffusibility;
(3) Back face provided with light diffusibility;
(4) Emission face provided with light-refractive unevenness;
(5) Back face provided with light-refractive unevenness.
Ways based on (1) provide uniform and highly effective emission with ease. However, the emission is subject to have a preferential direction much inclined with respect to a frontal direction. (Usually, the inclination is about 60 to 75 degrees to a normal with respect to the emission face.) Accordingly, a member (prism sheet) for modifying the inclined direction to the frontal direction must be arranged. Although employment of a light diffusion sheet brings some increase in frontal emission, it involves a wide light diffusion which leads to reduction in light energy efficiency.
Ways based on (2) or (3) hardly provide uniform and effective emission. The emission is also preferentially directed to oblique directions as in the case of (1). An increased light diffusibility checks the efficiency because of factors such as wide range scattering or absorption by light scattering elements (e.g. white ink).
Ways based on (4) are capable of causing light to escape from the emission face with ease while positive direction conversions are less effected. Accordingly, it is hardly expected to realize a highly efficient emission. In particular, it is not advantageous that they fail to generate light which travels from the back face to the emission face.
Ways based on (5) positively generate light which travels from a back face to an emission face of a light guide plate, being free from wide range light scattering. Accordingly, the ways are latently capable of effectively generating an emission directed to approximately frontal directions. However, in practice, prior arts fail to control propagating direction of emission sufficiently.
FIG. 1a to FIG. 1c illustrate examples to which the above (5) is applied. Referring to the illustrations, reference number 1 indicates a light guide plate made of a transparent material such as acrylic resin, which has a side end face to provide an incidence end face 2. A primary light source L is disposed beside the incidence end face 2 to be supplied with light from the primary light source L. One of two major faces 3, 4 of the light guide plate 1 provides an emission face 3. The other major face (called xe2x80x9cback facexe2x80x9d) is provided with a great number of recesses 5 with slopes 5a, 5b in profile.
The primary light source L emits light, which is introduced into the light guide plate 1 through the incidence end face 2. Upon encountering a recess, the propagation light within the light guide plate 1 (as represented by G1, G2) is inner-reflected by one slopes 5a to be directed to the emission face 3. Inner-incidence angle is denoted by xcex8 and an emission derived from beams G1, G2 is denoted by G1xe2x80x2, G2xe2x80x2. In other words, the slope 5a, which is relatively near to the incidence end face 2 (or primary light source L) compared with the other slope 5b, provides an inner-reflection slope for direction conversion. This effect is sometimes called edge-lighting effect.
The recesses 5 are formed like dots or linear channels. As shown in FIGS. 1a to 1c, formation pitch d, depth h or slope inclination xcfx86 of the recesses 5 is varied depending on distance from the incidence end face 2. Such variations prevent brightness on the emission face 3 from varying depending on distance from the incidence end face 2.
However, prior arts as shown in FIGS. 1a to 1c are subject to the following problems.
1. There is a region which is located behind the slope 5b as viewed from the incidence end face 2 and is hardly supplied with light. Therefore, a reduced formation pitch d gives no increasing in direction conversion efficiency, with the result that the emission face 3 is apt to show an unevenness in brightness.
2. Direction conversion is applied to only light that reaches the reflection surface (slope 5a), so to speak, directly, as G1, G2 shown in FIG. 1a, because direction conversion relies on only a single reflection (slope 5a) and no means for promoting incidence to the reflection surface (slope 5a) exists in the vicinity of the reflection surface. Consequently, it is difficult to realize an increased direction conversion efficiency.
The present invention aims to overcome the above-mentioned problem s of prior arts. That is, an object of the present invention is to improve an light guide plate used for emitting light, which is introduced from a side end face (incidence end face), from an emission face so as to have no region which light is hard to reach and to have an emission direction easily controllable and further to have a high direction-conversion efficiency.
Another object of the present invention is provide a surface light source device which capable of providing an illumination output light directed around a frontal direction without need of a direction modifying member such as prism sheet by employing the light guide plate improved as above.
The present invention further aims to improve the surface light source device as to have an increased light using efficiency and a reduced fine-bright-dark-unevenness which would otherwise appear corresponding to a distribution of micro-reflectors by means of improved characteristics of the light guide plate.
A still another object is to provide a liquid crystal display which allows a comfortable observation from a desired direction by applying the surface light source device to a backlighting arrangement or frontlighting arrangement for the liquid crystal display.
The present invention solves the problems by forming a great number of micro-reflectors, each of which includes gentle and steep slopes having a inner reflection function, on a back face of a light guide, wherein a double reflections effected by the slope pair is adopted as a main means for applying direction conversion to light.
In the first place, the present invention improves a light guide plate that comprises an emission face provided by a major face, a back face provided by another major face and a side end face provided by a minor face, light inputting being done through said side end face and light outputting being done through said emission face.
According to the improvement, said back face is provided with a great number of micro-reflectors for conversion of an inputted light and each of said micro-reflectors includes a first slope and a second slope, said first slope being inclined with respect to an extending plane of said emission face more gently as compared with said second slope, wherein said inputted light is converted into an inner output light directed to said emission face by a pair of inner reflections effected by said first slope and then effected by said second slope, thereby being emitted from said emission face.
Since the light guide plate has the first slope gently inclined before (on the input side) the second slope producing an inner output light directed to the emission face, much light is guided smoothly to the second slope via the first slope. In addition such a main route, there is a subsidiary route which brings a direct incidence to the second slope (without via the first slope), with the result that an effective direction conversion is realized by the micro-reflectors.
It is noted that many cases show light coming directions which vary depending on position on the back face. To fit such situation, orientations of said great number of micro-reflectors may vary depending on position on said back face.
The present invention provides an improved surface light source device that employs the above light guide plate. The present invention improves a surface light source device comprising a light guide plate which has an emission face provided by a major face, a back face provided by another major face and a side end face provided by a minor face, light inputting being done through said side end face and light outputting being done through said emission face, the surface light source device further comprising at least one primary source for said light inputting.
According to the above features of the light guide plate, said back face is provided with a great number of micro-reflectors for conversion of an inputted light and each of said micro-reflectors includes a first slope and a second slope, said first slope being inclined with respect to an extending plane of said emission face more gently as compared with said second slope, and said inputted light being converted into an inner output light directed to said emission face by a pair of inner reflections effected by said first slope and then effected by said second slope, thereby being emitted from said emission face.
Since the light guide plate has the first slope gently inclined before (on the input side) the second slope producing an inner output light directed to the emission face, much light is guided smoothly to the second slope via the first slope. In addition such a main route, there is a subsidiary route which brings a direct incidence to the second slope (without via the first slope), with the result that an effective direction conversion is realized by the micro-reflectors.
As forementioned, many cases show light coming directions which vary depending on position on the back face. To fit such situation, orientations of said great number of micro-reflectors may vary depending on position on said back face.
For example, if an employed primary light source is a point-like light source, said great number of micro-reflectors are orientated preferably according to an orientation distribution such that said first reflection slope accepts said inputted light travelling radially.
It is noted that some leaking light is produced because a direct incidence to the second slope (i.e. subsidiary route) hardly satisfy the total reflection condition. Much of such leaking light can be recovered by a reflection member disposed along the back face of the light guide plate. Such light recovered by reflection returns into the light guide plate and has chances of emitting at various positions (on the emission face).
As a result, not only loss of light is avoided but also fine-unevenness in brightness, which is apt to appear on the emission face, is relaxed. The reflection member preferably has a moderate irregular reflectivity.
Said light inputting may be done from a plurality of directions different from one another. In this case, said great number of micro-reflectors are preferably classified regarding orientation into groups corresponding to said plurality of directions respectively so that micro-reflectors belonging to each of said groups has charge of a partial production of said inner output light according to a share allotted to each of said groups.
A surface light source device improved according to the above manners may be applied to a back-lighting-type LCD having a LCD panel illuminated from the back side or to a front-lighting-type LCD having a LCD panel illuminated from the front side.
front-lighting arrangement the light guide plate, still further relating to a liquid crystal display employing the surface light source device for back-lighting or front-lighting.
If so applied, the LCD reflects the characteristics of the surface light source device. Accordingly, the LCD in accordance with the present invention provides a display screen that looks bright as viewed from a desired direction or position.